1. Field of the Invention
The present invention relates to a photoelectric conversion element, an image reading device, an image forming apparatus, an image reading method, and image forming method.
2. Description of the Related Art
In an image reading device which reads a document, CCD is conventionally mainly used as a photoelectric conversion element which performs photoelectric conversion of reflected light from the document. Recently, a CMOS linear image sensor attracts attention as the photoelectric conversion element by request for higher speed and lower power consumption of the image reading device. The CMOS linear image sensor is the same as the CCD in that this performs the photoelectric conversion of incident light by using a photo diode. However, although the CCD transfers a charge by a shift register and performs charge-voltage conversion by a charge detection unit after the transfer, the CMOS linear image sensor performs the charge-voltage conversion in the vicinity of a pixel to output to a subsequent stage.
In the conventional CMOS linear image sensor, a photo diode (PD) which performs the photoelectric conversion, a floating diffusion (FD) which performs the charge-voltage conversion of the charge accumulated by PD, a reset circuit which resets potential of FD, and a source follower (SF) which buffers a voltage signal of FD to transmit to the subsequent stage are formed in the pixel. That is to say, the conventional CMOS linear image sensor has a problem that an area (opening) of the photo diode is limited by a portion other than the photo diode (pixel circuit) arranged in the pixel and sensitivity thereof is deteriorated.
This problem is especially remarkable in the CMOS linear image sensor for reduction optical system in which a pixel size is small as compared to a contact image sensor (CIS) for same-magnification optical system even in a case of the same CMOS linear image sensor.
As a technical example to solve the above-described problem, Japanese Laid-open Patent Publication No. 2010-135464 discloses a CMOS solid-state imaging element in which a plurality of pixel rows in which a plurality of pixels is one-dimensionally arranged is arranged, the solid-state imaging element provided with a first pixel circuit separately provided for each pixel arranged in each pixel and a second pixel circuit provided so as to be shared by the pixels of each column, the second pixel circuit arranged outside the pixel column.
However, the conventional CMOS linear image sensor has a problem that the sensitivity is still low because a part of the pixel circuit is formed in the pixel. There also is a problem that only one of R, G, and B may be read at once and higher speed cannot be easily realized because the pixel circuit has a common part among colors (R, G, and B).
In view of above conventional problems, there is a need to provide a photoelectric conversion element, an image reading device, an image forming apparatus, an image reading method, and an image forming method capable of preventing deterioration in sensitivity.